Preparation of light sensitive surfaces



Dec. 11, 1956 e. LEWIN 2,773,730

PREPARATION OF LIGHT SENSITIVE SURFACES File d Dec. 17, 1953 INVENTOR 622M720 Z'W/N BY 1 Q MM \4 6mm;

ATTORNEYS United States Patent M PREPARATION OF LIGHT SENSITIVE SURFACES Gerhard Lewin, West Orange, N. J., assignpr to Tung- Sol Electric Inc., Newark, N. J., a corporation of Delaware Application December 17, 1953, Serial No. 398,695

9 Claims. (Cl. 316-8) The present invention relates'to the preparation of cesium antimonide and more particularly to the preparation of surface layers of such compound for use in photoelectric tubes, photomultipliers and the like and comprises an improved method of forming electron emissive surfaces which overcomes difliculties heretofore experienced in the manufacture of tubes of the above indicated type and substantially reduces the number of defective tubes ordinarily produced by methods heretofore employed.

The preparation of a cesium antimonide surface on a photo cathode or dynode involves the preliminary evaporation of antimony on to the parts to be surfaced and then, after the parts have been mounted within a tube and degasified by a vacuum bake, the subjection of the deposited antimony to cesium vapor While the tube is heated and connected to the vacuum system. Cesium antimonide is thus formed and excess cesium is removed by distillation into the colder parts of the system. In order to prevent decomposition of the formed antimonide and consequent malfunctioning of the tube, great care must be taken to avoid continued heating after all of the free cesium has been removed. On the other hand, unless all free cesium is removed, excessive electrical leakage will result when the tube is in use. In either case' h part of the exhaust tubulation of the tube or be attached to the manifold connected to a number of tubes being simultaneously processed. The cesium source may be in the reservoir or in the tube or two or more sources of cesium may be provided. During the period when the tube and reservoir are maintained at the temperature difierential, they are isolated from the vacuum system, either by tipping off the exhaust tubulation beyond the reservoir, or by shutting off the exhaust system from the manifold depending upon the location of the reservoir. The temperature differential is maintained in such manner as to keep the cesium pressure in the tube below the equilibrium value for the liquid and vapor phases and above the dissociation pressure of the cesium antimonide at the higher temperature. In practice it has been found that the difference in temperature should be relatively large. For example, 240 C. for the tube and 150 C. for the reservoir. These temperatures correspond to equilibrium pressures of 300 microns and 10 microns, respectively, whereas the dissociation pressure of cesium antimonide at 240 C. appears to be between 1 and 3 microns. The large temperature differential is important because the cesium tends to adhere to insulating surfaces more than to liquid cesium. After the excess cesium has been collected in the reservoir, the reservoir when forming part of the exhaust tubulation, is tipped off from the tube. When the reservoir is connected to the manifold, the tubes are tipped off from the manifold after the excess cesium has been removed.

The new method above briefly described will be better understood by reference to the attached drawing of which Fig. 1 represents a photomultiplier tube in the process of having the cathode and dynodes thereof surfaced with cesium antimonide, and

Fig. 2 illustrates a plurality of photomultiplier-tubes being simultaneously processed.

In Fig. 1 a conventional photomultiplier 2 is illus trated as provided with the usual cathode 4 and dynodes 6. The exhaust tubulation of the tube 2 is provided with an enlarged section 10 providing a reservoir for collection of excess cesium. A cesium activator 12 is mounted in the reservoir 10 and a second activator 13 is mounted on a support within the tube 2. A branch tube 14 connects to the vacuum system (not shown). The activators 12 and 13 are of conventional form. Each may comprise a perforated nickel envelope containing cesium chromate and silicon housed in a capsule. When the capsules are heated, as by radio frequency heating means, the cesium therein is vaporized and issues through the perforations in the nickel envelopes. Before the cathode and dynodes are assembled within the tube, antimony is vaporized on to their surfaces. The parts are then assembled within the tube and a preliminary baking to degasify the electrodes is done under vacuum conditions, that is, with the branch 14 connected to the vacuum system. The activators 12 and 13 are then flashed and the tube 14 tipped off to isolate the reservoir 10 and tube 4 from the vacuum system. The tube is then heated to a temperature of about 240 C. and the reservoir 10 is heated to a temperature of about 150 C. and this temperature differential is maintained for from 15 minutes to an hour, preferably for about a half an hour. During this baking the cesium vapor reacts with the antimony layer on the cathode and dynodes to form cesium antimonide. Excess cesium is drawn into the reservoir 10 because of the lower temperature maintained in that part of the system. After the heating period is over, the parts are allowed to cool. Preferably the temperature differential is maintained during the cooling period or at least until the temperature of the reservoir has fallen to about C. The tubulation 8 is then sealed oif adjacent the photomultiplier tube and the process is complete. Although two activators have been described, one in the tube and one in the reservoir, either could be omitted as activation of the cesium source, wherever located, will release sufiicient cesium to react with the antimony layers on the electrodes and maintenance of the temperature differential will insure removalof excess cesium from the tube and collection thereof in the reservoir.

Fig. 2 illustrates simultaneous processing of a number of tubes. In this alternative arrangement a reservoir 16 containing liquid cesium or one or more activators is connected through a valve or stopcock 18 to the manifold 20 of a conventional exhaust system. A valve 22 is provided between the exhaust line 24 and the manifold 20. The tubes 28, each of which contain electrodes upon which surface layers of antimony have been vaporized are connected to the manifold 20 by their exhaust tubulations 30. The manifold and tubes are contained within a suitable oven, symbolized by the enclosure 29 and the reservoir 16 is similarly positioned Within an oven, symbolized by the enclosure 31; alternatively the tubes, manifold and reservoir could be within a single oven having means for controlling the temperature at different parts there of.

,If the reservoir 16 contains one or more activators, valves 18 and 22' are opened and preliminary baking and degasifying is first made as in conventional practice. The

. activator or activators in the -reservoir is or arerthen flashed and thereafter valve 22 is closed. The tubes -:28 and parts associated therewith are then'heated to about 240 C. and the reservoir'16 is heated to about 150 C. and this temperature differential is maintained for about one-half hour as inthecase ,of the processing of the single tube as describedin connection .with Fig. 1. After the excess cesium 'hascollected in the reservoir 16, the tubes and reservoir are allowed to cool while the temperature difiFerential is maintained. The valve 18 is then closedand the individual tubes are tipped off at their exhaust tubulations 30. If the reservoir 16 containsliquid cesium rather than activators,-the same process is followed except that valve 18 is maintained closed during the preliminary-baking and degasifying step. It will be understood that the valves 18 and 22 should be of material which 'will withstand'the corrosive action of cesium vapor and the high temperature of the bake.

The invention has now been described with particular reference to the forming of cesium antimonide on the cathode and dynodes of photomultiplier tubes. Obviously the same method is applicable to the formation of such surfaces on cathodes or two-electrode .photoelectric tubes or wherever a light sensitive surface is to be formed.

To avoid removal of the cesium vapor by getter action, a neutral glass, such as lime glass is preferably employed for the exhaust tubulation of the tube the elements of which are to be provided with the electron emissive surface. Obviously'variationsin the process as heretofore described can be made without departing from the spirit of the invention. Although it has been found that the particular temperatures mentioned are suitable for commercial practice of the method, the specified temperatures may be varied somewhat, say plus or minus per cent. It is essential, however, that the pressure be maintained above the dissociationpressure of the cesium compound for the temperatures employed and that a relatively large temperature differential be maintained to insure removal and collection ofthe excess cesium.

The following is claimed:

1. The method of forming on an element a surface layer of cesium antimonide which comprises depositing on the element a layer of antimony to be combined with cesium, mounting the element in an enclosure having a reservoir communicating therewith, degasifying theelement by vacuum baking, then introducing cesium vapor into the enclosure for formation of the antimonide, heating the reservoir to one temperature'and the element to a higher temperature to maintain the pressure of the cesium vapor less than the equilibrium pressures of cesium vapor and liquid at such temperatures but above the dissociation pressure of cesium antimonide at the higher temperature whereby excesscesium is collected in the reservoir, cooling the element and the reservoir and finally isolating the enclosure from the'reservoir.

2. The method according to claim 1 wherein the temperature dilterential between the element arid-the reservoir is about 90 C. and is maintaineduuringcoolin prior to isolation of the enclosure from the'reservoir.

3. The method accordingto claim v-2 wherein the .element is heated to a temperature of about 240 C. and the reservoir to a temperature of about 150 C. and these temperatures are maintained for a period between minutes and an hour prior to cooling.

4. In the manufacture of a light sensitive tube th method of forming an electron emissive surface on a tube 'ele'ctro'de whichcomprises'vaporizing antimony "onto the electrode, assembling the electrode with the other tube elements withina tube provided with an exhaust'tubulation and with a reservoir communicatin the'rewith,fexhausting the tube while baking to degasify the tube elements and the electrode carrying the antimony :surface, introducing cesium vapor into the tube, tubulation and reservoir, isolating the tube, tubulation andreservoir from the exhaust system, heating the tube and reservoir, to difierent temperatures to maintain the tube at a higher temperature than the reservoir to thereby collect excess cesium intherreservointhen cooling the tube and reservoir while maintaining the temperature differential and finally isolating thetube from the 'reservoir by tipping off the exhaust =tubulation between the reservoir and the tube.

5. The-method'according to claim 4 wherein the tube is heatedto a temperature of about 240 C. and the reservoir to a temperature of about 150 C.

6. The-methodaccording to claim 4 wherein the tem-? peratures of the tube and reservoir are maintained sub-' stantially constant for about one-half'hour before cooling.

7. The method according to claim 4 wherein the tube is'heated to about 240 C-., the reservoir to about 150 C, and*cooling'is allowed to continue until'the respective temperatures'are about 170 C. and C. before isolation of the tube from the'reservoir. 1

8. The method of forming electron emissive surfaces. on tube elements which comprises vaporizing antimony on to the elements, assembling the elements within their respective tubes, coupling the tubes to a vacuum system and-to areservoir, baking the tubes while connected to the reservoir and to the vacuum system to removegas, then introducing cesium vapor-into the tubes and reservoir forreaction with the antimony surfaces on the elements,-isolating the reservoir and tubes from the vacuum system and then heating the tubes at onetemperature and the reservoir to a lower temperature to collect in the reservoir excess cesium from the cesium antimonide formed on the tube elements.

I 9. Themethod according to claim 8 wherein the tubes are heatedto about 240 C. and the reservoir to about C. for about one-half hour and then allowed to cool until therespective temperatures reach about C. and 80 C. and finally sealing off the tubes from the reservoir and from each-other.

jPresc'ot't Nov. 2, 193 7 'DeBoer etal. Iu'ne'6, 1939 Sommer NOV. 14, 1950 2097,46? 2.161545 2;529,sss 

